The formation of an oil and gas well includes securing a metal casing within a wellbore via cement forming an annular structure between the sidewall of the wellbore and the outer diameter of the casing. Downhole acoustic tools may be utilized for cement bond logging (CBL) to evaluate bonding quality between the casing and the cement, such as by evaluating amplitudes of casing arrivals traveling from a transmitter to the casing and refracted to a sensor axially separated from the transmitter. Downhole acoustic tools may also or instead be utilized for radial bond logging (RBL) to evaluate azimuthal variation of the cement bonding, such as by evaluating casing arrivals across sensors at various azimuthal locations around the downhole acoustic tool.
However, CBL and RBL both resort to casing arrival amplitudes, which are sensitive to the position of the downhole acoustic tool within the casing. Consequently, eccentering of the downhole acoustic tool from the axis of the casing perturbs casing arrival amplitudes, which can result in inaccurate interpretation of the cement bonding quality.
Moreover, the measurement frequency range of acoustic logging tools are generally below 30 kilohertz (kHz), including for evaluating formation elastic properties in uncased, open-holes, and for evaluating cement bond quality in cased-holes. This frequency range limits the spatial resolution with which the acoustic logging tools can provide elastic properties of formations/muds in open-holes, and cement bonding quality around the casing in cased-holes.
To evaluate elastic properties of a formation having intrinsic and/or stress-induced anisotropy, open-hole acoustic logging may utilize monopole or multipole (e.g., dipole or quadrupole) logging. However, in azimuthally heterogeneous formations (e.g., bedding intersecting the wellbore in a horizontal well), logging results are less accurate, because averaged elastic properties are measured around the wellbore. Unipole logging may also be utilized, with an azimuthally focused (or localized) transmitter placed on one side of the tool, and with waveforms measured on the same side of the tool. However, because the transmitter and the array receivers are typically placed on just one side of the tool, the tool is rotated to provide azimuthally full coverage around the wellbore, which is possible with logging-while-drilling (LWD) implementations of the acoustic tool but not wireline and other non-LWD conveyance means, because non-LWD conveyance means generally cannot be manipulated at the surface in a manner permitting accurate rotational positioning of the acoustic tool.
To evaluate casing cement in cased-holes, the acoustic logging may utilize a frequency of about 100 kHz. However, the impact of tool eccentering becomes larger because such logging is based just on amplitudes of measured casing signals. With lower frequencies, and/or in the presence of a second casing or a formation having a compressional slowness less than about 200 microseconds per meter (μs/m), additional inaccuracies may occur due to the acoustic signal wavelength not being sufficiently smaller than the features intended to be detected.